Method and a device for detecting slag

ABSTRACT

The invention relates to a method and a device of detecting the presence of slag in a shroud ( 4 ) through which molten metal passes from a ladle ( 2 ) to a tundish ( 6 ). A transmitting coil ( 20 ) generates an electromagnetic field. An induced voltage generated at a receiving coil ( 24 ) is compared with a defined voltage range. If the induced voltage has a value outside the defined voltage range it is indicative of the presence of slag. The coils ( 20,24 ) are arranged in an unmovable manner in relation to the shroud. The invention also relates to a casting plant.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and a device for detectingslag in a flow of molten metal. In particular, the invention relates todetecting the presence of slag by means of an electromagnetic fieldwhich is propagated through the molten metal. The invention also relatesto a casting plant comprising such a device.

BACKGROUND OF THE INVENTION

In the metallurgical industry there are different processes in whichliquid metal is to be processed in one way or another. One example isthe casting of metal, such as steel. In part of such a casting process,the liquid metal is supplied from a ladle to a tundish via a pouringnozzle which is generally referred to as a shroud. The metal flows fromthe tundish via another pouring nozzle to a casting mould or chillmould, in which the metal is cooled and transformed into solid form.

In smelting and heating processes there is a risk of slag being formedin the molten metal. In order to minimize the risk of slag beingincluded in the final cast product, the casting plant may include a slagdetection device. If the slag detection device detects slag in themolten metal an alarm is generated so that suitable measures may betaken.

In the prior art one type of slag detection device includeselectromagnetic coils placed above a sliding gate in the ladle. Thistype of slag detection device works on the principle that for anelectromagnetic field the penetration depth of metal is much less thanthe penetration depth of slag. Thus, a large portion of theelectromagnetic field is allowed to pass through slag, similarly to air.

Even though the prior art detecting device generally providessatisfactory detecting results, it has some drawbacks. One drawback isthat every ladle used in a casting plant has to be equipped with theelectromagnetic coils, the coils being welded to the ladle. Furthermore,the coils installed in a ladle are in such a position where there is arelatively high risk of mechanically damaging the sensors. Thus, itwould be desirable to install electromagnetic slag detecting sensors(coils) in such manner that cuts costs and minimizes the risk ofmechanical damage to the sensors.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve a method, a device anda casting plant which alleviate the drawbacks of the prior art.

This and other objects, which will become apparent in the following, areaccomplished by a method, a device and a casting plant as defined in theaccompanied claims.

The invention is based on the insight that by optimising the constancyof the propagation of an electromagnetic field through a flow of moltenmetal, it is possible to, in contrast to the prior art, move theelectromagnetic slag detection out of the ladle. In particular, bycontrolling the molten metal to be subjected to a positionally constantelectromagnetic field, it is even possible to perform the slag detectionat such location which moves as the molten metal passes said location.In fact, it has been found that a shroud which guides molten metal fromthe ladle to a tundish can be subjected to electromagnetic slagdetection, in a simple and economical manner, despite the movements ofthe shroud. Movement of the shroud may occur during the pouring of themolten metal. Such movement may be linear or in arcs of a circle. Theshroud also tends to move when the sliding gate at the ladle is moved.

Consequently, according to one aspect of the invention, there isprovided a method of detecting the presence of slag in a channel forguiding molten metal. According to this method a transmitting coil isused for generating an electromagnetic field which enters the channeland its contents. A receiving coil is used to receive theelectromagnetic field that has entered the channel and its contents. Theelectromagnetic field generates an induced voltage in the receivingcoil. The value of the induced voltage, e.g. if it is outside or withina defined voltage range, is indicative of the presence or absence ofslag in said contents. The electromagnetic field generated by thetransmitting coil(s) is arranged to enter a shroud and its contents, theshroud being the above-mentioned channel for guiding molten metal, inparticular from a ladle to a tundish. The transmitting and receivingcoils are kept substantially unmovable relative to the shroud. Thus, themethod enables detection of the presence of slag in the shroud.

According to a second aspect of the invention, there is provided adevice for detecting slag in line with the above mentioned method.

The transmitting coil and the receiving coil are suitably held in placeby either a common or separate coil holders. If a common coil holder isto be utilised it may be designed in the form of a fork having at leasttwo branches. The transmitting coil and the receiving coil may then beheld by respective branches adapted to be placed on respective sides ofthe shroud. The forked coil holder may either stand by itself or beconnected to other equipments as will be explained below.

During the process of slag detection the coils are kept unmovablerelative to the shroud. If the coils are unmovable relative to theshroud, i.e. they follow any movement of the shroud, an even signal isobtained for an unchanged flow of molten metal. This may be accomplishedby e.g. mounting the transmitting coil and the receiving coil torespective branches of a forked coil holder mentioned above. In thatcase, the forked coil holder is suitably placed in such manner that animagined straight line drawn between the transmitting coil and thereceiving coil crosses the shroud. In other words the shroud is placedbetween the branches of the forked coil holder. An advantage of using aforked coil holder is that it is possible to use a relatively long stemthereof, wherein positioning of the coils around the shroud can beaccomplished by manoeuvring at a distance from the shroud without havingto access the shroud closely. The branches of the forked coil holderfacilitate correct positioning of the coils and reduces the risk ofrelative movements between the coils thereby avoiding unnecessaryerroneous signal variations. The risk of relative movements between thecoils are further reduced if the branches are made relatively short.Thus, a relatively long stem having short branches may suitably be used.Since the forked coil holder, preferably, does not entirely surround thecircumference of the shroud it is easily locatable to and removable fromthe shroud.

In order to enable the coils to substantially follow any movement of theshroud, the forked coil holder is suitably suspended to an arrangementthat follows the movements of the shroud. One example of such anarrangement is a shroud manipulator of any known type. A shroudmanipulator is. generally in the form of an elongate arm that is used tofetch, position and support the shroud. Such a shroud manipulator maycomprise a holder portion, e.g. in the form of a ring or horse shoe, inwhich the shroud is suspended. The shroud manipulator may also comprisea gripping means, e.g. in the form of a fork-shaped part, carrying theholder portion of the shroud and allowing the holder portion to move sothat the shroud can be in a vertically suspended state during casting,irrespectively of the vertical position of the connection point of theshroud to the ladle. The gripping means is generally attached to an armwhich may be automatically or manually controlled. Such a shroudmanipulator arm is usually connected to a unit which can make the armmove vertically, horizontally and in rotational movement. This isadvantageous since the shroud manipulator is used for fetching a shroudfrom a location spaced from the tundish. Somewhat horizontally inclined,the shroud is moved from said location by means of the shroudmanipulator which has been rotated for achieving the inclination of theshroud. The inclined shroud can thereby be brought into the spacebetween the ladle and the tundish, and by rotating the shroudmanipulator arm the shroud can be brought into a fully vertical positionand the shroud is placed against the outlet of the ladle. The shroud maybe kept in place by pressing the shroud manipulator upwardly. Duringcasting the ladle may be raised or lowered, thereby causing the shroudto move relative to the shroud manipulator in the vertical plane of theextension of the shroud manipulator. This change of ladle position isgenerally not carried out when the relevant slag detection is to beperformed at the end of the emptying of the ladle.

The forked coil holder may thus be mounted to such a shroud manipulator.As an alternative, the forked coil holder may be suspended to thesliding gate unit mounted on the ladle, and other alternatives are alsopossible in order to achieve the desired motion following effect.

The advantage of using an already existing shroud manipulator forproviding support to a coil holder arrangement becomes clear from athird aspect of the invention.

Thus, according to the third aspect of the invention the coil holder isplaced on a manipulator, in particular a shroud manipulator, the coilsbeing unmovable relative to the shroud at least in a horizontaldirection perpendicular to the extension of the shroud manipulator. Inparticular, the third aspect of the invention provides a method ofdetecting the presence of slag in a shroud for guiding molten metal froma ladle to a tundish, the method comprising: providing a forked coilholder having at least two branches; mounting at least one transmittingcoil to a first branch and at least one receiving coil to a secondbranch of the forked coil holder; mounting said forked coil holder to ashroud manipulator adapted to manipulate the shroud; placing the forkedcoil holder in such manner that an imagined straight line drawn betweenthe transmitting coil and the receiving coil crosses the shroud;generating, by means of said transmitting coil, an electromagnetic fieldthat enters the shroud and its contents; generating an induced voltageby means of said receiving coil which is subjected to theelectromagnetic field having entered the channel and its contents,wherein said induced voltage is indicative of the presence or absence ofslag in said contents; and keeping the coils unmovable relative to theshroud.

The forked coil holder may be mounted to the shroud manipulator when themanipulator has already located the shroud in place between the ladleand the tundish. Alternatively, the forked coil holder may be mounted tothe shroud manipulator before the manipulator has arranged the shroudinto correct position, or even at a point in time when the shroud hasnot yet been connected to the shroud manipulator. The forked coil holderis suitably connected to an arm of the shroud manipulator.

The advantage of a method according to the third aspect of the inventionis i.a. that it is place-saving, since use is made of the shroudmanipulator as a carrier for the coils or the coil holder. Anotheradvantage is that the coils may be arranged in a fix relation to theshroud, due to their fix relation to the shroud manipulator which duringcasting holds the shroud.

The shroud manipulator is also included in a device or system accordingto a fourth aspect of the invention. Thus, according to the fourthaspect of the invention a device is provided for detecting the presenceof slag in a shroud for guiding molten metal from a ladle to a tundish,comprising: at least one transmitting coil for generating anelectromagnetic field to be entered into the shroud and its contents; atleast one receiving coil for receiving the electromagnetic field thathas entered the shroud and its contents, and for generating an inducedvoltage, wherein said induced voltage is indicative of the presence orabsence of slag in said contents; a shroud manipulator adapted tomanipulate the shroud; a forked coil holder mounted to the shroudmanipulator and having at least two branches, a first branch carryingsaid at least one transmitting coil and a second branch carrying said atleast one receiving coil, the two branches being placeable in suchmanner that the shroud is located between them and that said at leastone transmitting coil and said at least one receiving coil are in astationary position in relation to the shroud.

The two branches that hold the transmitting coil and the receiving coilrespectively may in at least one embodiment of the invention beelectrically isolated from one another. If for example the branches areconnected to the stem by means of a common cross bar, that bar may beprovided with electrical isolation. This type of interruption minimizesunwanted electromagnetic fields.

Even though the forked coil holder provides many advantages, the methodaccording to the first aspect of the present invention, could also becarried out by arranging the coils on separate holding elements, e.g.separate arms, that are electrically isolated from one another. Suchseparate holding elements may also be mounted to the shroud manipulatoror other shroud motion following arrangement. Accordingly, there aredifferent ways in which to generate an electromagnetic field by means ofone or more transmitting coils from one side of the shroud and toreceive said electromagnetic field by means of one or more receivingcoils on another side of the shroud.

According to at least one embodiment of the first and second aspect ofthe invention, as an alternative to providing transmitting and receivingcoils on different sides of the shroud, each coil is arranged tosurround the shroud. The coils may be given a toroid or annular form andthe shroud will extend through each toroid. The coils may be mounted tothe shroud by means of a coil holder arrangement including fasteners forsecuring the coils to the shroud. Alternatively, the annularconfiguration can be mounted to some other location, such as the slidinggate at the ladle, and be suspended therefrom so as to surround theshroud. Another alternative is to mount the coils between the branchesof a forked coil holder which in turn is mounted to a shroud manipulatoror other shroud following arrangement.

It should be clear from the above discussion that regardless of the typeof coil arrangement chosen, it is possible to arrange the coils in suchmanner that the coils are enabled to substantially follow any positionalvariations of the shroud. As previously explained, the advantage of thisis that the propagation of the generated magnetic field entering theshroud will be constant, wherein the accuracy of the detection isincreased even though the shroud moves during the pouring of the moltenmetal.

The voltage induced by the receiving coil(s) is used to determinewhether or not slag or slag is present. According to at least oneembodiment of any aspect of the invention, any induced voltage having avalue outside a defined voltage range is indicative of the presence ofslag. The flow of molten metal passing through the shroud is determinedand the voltage range is defined depending on the magnitude of thedetermined flow of molten metal. The advantage of this is that thedetection of slag in the shroud may be even more accurate. A shroud isnot necessarily completely filled with molten metal at all times. Sincegas and slag have similar influence or lack of influence on theelectromagnetic field, some gas in the shroud could be taken for slag.According to said embodiment an alarm limit is set depending on theamount of gas (e.g. air) present. In particular, the embodiment is basedon the insight that the presence of gas can be determined by determiningthe flow, i.e. the volume per unit of time, of molten metal passingthrough the shroud.

The advantage of determining the flow of molten metal is that itprovides a ratio of the flow components, i.e. the amount of metal to theamount of gas in the shroud, wherein depending on a change in said ratiothe sensitivity of the detection device may be varied. For instance, ifthe flow of molten metal decreases after a while, the requirement for orlevel of providing an alarm of the detection device is increased. Thisis done by increasing said defined voltage range. This means that agiven induced voltage may be either indicative of the presence of slagor not depending on the presently defined voltage range. Thus, on theone hand a given induced voltage value may fall outside a voltage rangedefined before a decrease of the flow, which would result in anindication of presence of slag and on the other hand, after the decreaseof the flow and the increase of the voltage range, the same value ofinduced voltage would be covered by the redefined voltage range, whereinit would be construed as presence of gas, if any, and not slag. The riskof slag being entrained in the flow of molten metal usually increasestowards the end of the process, when the ladle is close to beingdrained.

The variation of voltage range may be illustrated in the following way.Suppose that the receiving coil, for an empty shroud, i.e. just gas andno molten metal, generates an induced voltage having a signal strengthV₁. When molten metal passing through the shroud completely fills up theshroud a voltage having a signal strength V₂ will be induced, whereinV₂<V₁. Thus, for a filled shroud the voltage range is set to at leastthe interval 0-V₂, however, in order to be on the safe side and not riska false alarm, the voltage range may be extended somewhat. When theshroud is only partly filled with molten metal, the definition of thevoltage range is suitably adjusted. The newly defined voltage range willbe 0-V₃, wherein V₂<V₃<V₁.

A simple calculation example will now follow. Suppose that for a shroudcompletely filled with molten metal the voltage range is defined suchthat the slag is only indicated as being present if the magnitude of theinduced voltage indicates a metal content of 90% or less (while thedetermined magnitude of flow of molten metal is still about 100%). Thus,if the induced voltage corresponds to a shroud filled by 95% with moltenmetal, there will be no slag indication. This provides a 10% safetymargin for avoiding false alarm. Now, suppose that later on it isdetermined that the flow of molten metal has decreased to half, i.e.only 50% of the shroud is filled with molten metal and the rest isfilled with gas. In order to maintain the 10% safety margin the voltagerange is defined such that the presence of slag is indicated if thevalue of the induced voltage corresponds to a metal content of 45% orless. Note that the numbers in the this example are merely exemplifyingin order to illustrate a general principle of varying the voltage range.

As previously mentioned, the flow of molten metal guided through theshroud may be determined either directly or indirectly. Different typesof flow meters may be employed for direct measurement of the flow insidethe shroud. However, it may be simpler to make use of an indirect methodof determination. For instance, the flow of molten metal passing throughthe shroud may be determined by measuring the teeming rate in thetundish and calculating the flow of molten metal from said measuredteeming rate. Another alternative is to measure the rate of decrease inweight of the ladle content and calculating the flow of molten metalfrom said measured rate of decrease in weight. Yet another alternativeis to measure the casting speed and the dimension of the cast productafter the chill mould, the speed and dimension being indicative of theflow. A further alternative is to detect the degree of opening or theposition of the sliding gate mounted under the ladle and to provide afeedback signal to a processor, which calculates the flow of moltenmetal from the positional information of the sliding gate, i.e. how muchthe siding gate has been opened. An advantage of these differentalternatives is that standard existing measuring systems may be used toprovide the relevant information desired to define slag alarm settings.

In at least one embodiment of any aspect of the invention, as asupplement to the control of the defined voltage range, the frequency ofthe electromagnetic field generated by the transmitting coil may becontrolled. It has been found that a higher frequency provides a morestable induced voltage signal when the shroud is not completely filledand a turbulent flow of molten metal is present inside the shroud. Thus,when a turbulent flow inside the shroud has been detected, the frequencyof the electromagnetic field is changed, suitably increased, by thesystem. Also, the magnitude of the flow of molten metal influences thechosen frequency in order to penetrate the metal and detect any slag inthe centre of or at any other place if the flow of molten metal.External frequencies that can interfere with the measurement can be aground for changing the frequency. Thus, the electromagnetic fieldgenerated by the transmitting coils(s) may be of alternatingfrequencies. Also, several coils may be used for generating severalelectromagnetic fields, each with a different frequency, in order tominimise noise effects. Furthermore, several coils may be used forgenerating several electromagnetic fields, one or more being ofalternating frequencies.

The transmitting coil that generates the electromagnetic field may beprovided with directional elements, such as a core, for optimiseddirection of the electromagnetic field towards the portion of the shroudin which it is intended to check whether or not slag is present.Similarly the receiving coil may be provided with such directionalelements.

Due to the high temperature environment, according to at least oneembodiment of the invention, the transmitting and receiving coils may becooled by means of a suitable cooling arrangement, such as coolingchannels containing water or gas.

According to a fifth aspect of the invention, there is provided a devicefor detecting the presence of slag in a shroud which guides molten metalfrom a ladle to a tundish, the device comprising a forked coil holderhaving at least two branches, a first branch carrying a transmittingcoil and a second branch carrying a receiving coil, the two branchesbeing placeable in such manner that the shroud is located between them.This enables the coils to be substantially unmovable relative to eachother and the shroud and the contained metal stream, thereby avoidingunwanted signal variations Another advantage is that the coils areindependent of the ladle.

According to a sixth aspect of the invention, there is provided acasting plant, which comprises a ladle, a tundish and a shroud betweenthem. The casting plant also comprises a device for detecting thepresence of slag inside the shroud, as has been previously described inthis application.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanieddrawings, in which:

FIG. 1 is a schematic illustration of a device arranged to detect slagin a shroud in accordance with at least one embodiment of the invention.

FIG. 2 is a more detailed schematic illustration of the components shownaround the shroud in FIG. 1.

FIG. 3 is a flow chart illustrating an example of an operation of acontrol unit.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a ladle 2 adapted to contain moltenmetal which is to be passed through a shroud 4 over to a tundish 6. Themetal flows from the tundish 6 via a pouring nozzle to a chill mould, inwhich the metal is cooled and transformed into solid form, the pouringnozzle and the chill mould not being shown in the figure. A forked coilholder 10 is provided at the outside of the shroud 4 and is mounted to ashroud manipulator 30. The forked coil holder 10 contains a transmittingcoil for generating an electromagnetic field to be propagated throughthe shroud and to be received by a receiving coil which is also locatedin the forked coil holder 10. The forked coil holder 10 and the coilswill be further described in connection with the discussion of FIG. 2.

Continuing with FIG. 1, a signal cable 40 extends between the forkedcoil holder 10 and a connector 42. The signal cable 40 contains internalcables or separate wires for leading signals to and from thetransmitting coil and the receiving coil, respectively. Also connectedto the connector 42 are a transmitter signal cable 44 and a receiversignal cable 46 containing wires for providing signals to and from thetransmitting coil and the receiving coil, respectively. The receiversignal cable 46 is at its other end connected to a preamplifier unit 48in order to amplify generated induced voltage signals from the receivingcoil. A preamplifier cable 50 passes on the signals to a control unit 52which comprises a processor. The transmitter signal cable 44 is directlyconnected to the control unit 52. The generation of an electromagneticfield by the transmitting coil can thus be controlled by sending asignal from the control unit 52, via the transmitter signal cable 44,the connector 42 and the associated wires in the signal cable 40 to thetransmitting coil inside the forked coil holder 10.

The control unit 52 is connected to an alarm unit 54 via an alarm cable56. The control unit 52 compares the received induced voltage signalfrom the receiving coil with a predefined voltage range. If the receivedvoltage is not included in said voltage range, the control unit 52activates the alarm unit 54, which may suitably generate an acousticand/or visual alarm. Also, even though not shown, a control panel may beprovided for enabling continuous visual monitoring of the slag detectionprocess.

Furthermore, the control unit 52 receives information about the flow,i.e. the volume per unit of time of molten metal through the shroud.This piece of information is received over a flow input line 58 which atits other end is connected to a flow determining device (not shown),which can be either a direct measuring device or an indirect measuringdevice with subsequent calculation, as previously explained. If it isdetermined that a significant change in flow of molten metal hasoccurred, the control unit 52 will set a different voltage range. Adecreased flow results in the setting of a larger voltage range and anincreased flow results in the setting of a narrower voltage range. Thus,the point at which the alarm unit 54 is activated is dependent on thereceived flow information.

In FIG. 2 the forked coil holder 10 and its suspension to the shroudmanipulator 30 holding the shroud 4 is shown in more detail. FIG. 2 is apartly cut away perspective view. The forked coil holder 10 comprises astem portion 12, a first branch portion 14, a second branch portion 16and a cross bar portion 18. In this embodiment the branch portions 14,16 are located at opposite ends of the cross bar portion 18 and the stemportion 12 is connected to the middle of the cross bar portion 18.However, in alternative embodiments the stem portion could be placed atone end of the cross bar portion and even be an elongation of one of thebranch portions.

FIG. 2 illustrates that the interior of the first branch portion 14comprises at its end farthest from the cross bar portion 18 atransmitting coil 20. The wires 22 of the transmitting coil are passedthrough the first branch portion 14, via the cross bar portion 18 andthe base portion 12 to the control unit (see also FIG. 1).

FIG. 2 also illustrates that the interior of the second branch portion16 comprises at its end farthest from the cross bar portion 18 areceiving coil 24. The wires 26 of the receiving coil 24 are passedthrough the second branch portion 16, via the cross bar portion 18 andthe base portion 12 to the control unit (see also FIG. 1).

The ends of the branches portions 14, 16 are so positioned that thetransmitting coil 20, the shroud 4 and the receiving coil 24 are locatedin a straight line. In other words, the shroud 4 is located between thetwo coils 20, 24. The branch portions 14, 16, or sections thereof, maybe controllable between different anales relative to the rest of theforked coil holder. This enables the coils 20, 24 to be correctlypositioned for setting the main direction of propagation of theelectromagnetic field at a right angle to the shroud in workingposition.

The wires of the two coils 20, 24 may be electrically isolated from oneanother in the cross bar portion 18 and the stem portion 12 in order toavoid unwanted electromagnetic influence on any signals.

The forked coil holder 10 is suitably made of mainly nonmagneticmaterial, such as any one of stainless steel, ceramic material, fibreglass or other similar suitable material that substantially does notinterfere with the electromagnetic field.

The forked coil holder 10 is suspended to the shroud manipulator 30 bymeans of a clamping arrangement 32 comprising two protrusions 34 each ofthem having at its end a clamp 36 surrounding the shroud manipulator 30.The clamps 36 are tightened to the shroud manipulator 30 by means ofbolts or screws 38.

The shroud manipulator 30 illustrated in FIG. 2 is of standard type andcomprises at the end portion that holds the shroud 4 a gripping means.The gripping means comprises two grip portions 62 that are inclinedupwards from the rear elongate portion 64 of the shroud manipulator 30.The inclination depends among other things on how the shroud is to bepositioned. The initial gripping point on the shroud differ from theshroud positioning point of the manipulator, making it desirable tochange the angle. The grip portions 62 are further connected to a holder66 shaped as a horse shoe. FIG. 3 is a flow chart illustrating anexample of an operation of the control unit 52 shown in FIG. 1.

In a step S1 the control unit defines a voltage range. In a step S2 thecontrol unit receives a signal indicative of the voltage induced at thereceiving coil. In a step S3 the control unit compares the value of thereceived signal with the defined voltage range and determines whether ornot said value is within the defined voltage range. If said value is notwithin the defined voltage range (in such a case the absolute value ofthe received signal is generally larger then the absolute value of themaximum voltage of the voltage range), then the control unit activates aslag detection alarm in a step S4. If on the other hand it is determinedin step S3 that the value of the received signal is encompassed by thedefined voltage range, the control unit returns to step S2 to receive anew signal to be compared in the subsequent step S3 etc.

In parallel to said steps, the control unit also receives a flow signalin a step S5. The flow signal is indicative of the volume per unit oftime of molten metal flowing through the shroud. In a step S6 thecontrol unit compares the determined flow of molten metal with storedvalues and examines whether or not there has been a significant changeof flow over time. If there has been no significant change the controlunit continues with the step S5 to examine subsequent flow signals.However, if there is a significant change, the control unit will inaccordance with the stored values (such as a table) redefine the voltagerange in the step S1.

It should be noted that FIG. 3 is only an example of the operation ofthe control unit for elucidating purposes and that other alternativesare also possible.

It should also be noted that numerous modifications and variations canbe made without departing from the scope of the present inventiondefined in the accompanied claims. For instance, even though thedescription is focused on the use of one transmitting coil and onereceiving coil, the method and device according to the invention maycomprise several transmitting coils and several receiving coils.

1. A method of detecting the presence of slag in a shroud for guidingmolten metal from a ladle to a tundish, comprising: providing a forkedcoil holder having at least two branches; mounting at least onetransmitting coil to a first branch and at least one receiving coil tosecond branch of the forked coil holder; mounting said forked coilholder to a shroud manipulator adapted to manipulate the shroud; placingthe forked coil holder in such manner that an imagined straight linedrawn between the transmitting coil and the receiving coil crosses theshroud; generating, by means of said transmitting coil, anelectromagnetic field that enters the shroud and its contents;generating an induced voltage by means of said receiving coil which issubjected to the electromagnetic field having entered the channel andits contents, wherein said induced voltage is indicative of the presenceor absence of slag in said contents; and keeping the coils unmovablerelative to the shroud.
 2. A method of detecting the presence of slag ina channel for guiding molten metal, comprising: generating, by means ofat least one transmitting coil, an electromagnetic field that enters thechannel and its contents; generating an induced voltage by means of atleast receiving coil which is subjected to the electromagnetic fieldhaving entered the channel and its contents, wherein said inducedvoltage is indicative of the presence or absence of slag in saidcontents; generating said electromagnetic field so that it enters ashroud and its contents, the shroud being said channel for guidingmolten metal from a ladle to a tundish, and by keeping the coilsunmovable relative to the shroud.
 3. The method as claimed in claim 2,further comprising: providing said at least one transmitting coil intoroid form and arranging it so as to surround the shroud, and providingsaid at least one receiving coil in toroid form and arranging it so asto surround the shroud.
 4. The method as claimed in claim 2, furthercomprising: providing a forked coil holder having at least two branches;mounting the transmitting coil to a first branch and the receiving coilto a second branch of e forked coil holder; and placing the forked coilholder in such manner that an imagined straight line drawn between thetransmitting coil and the receiving coil crosses the shroud.
 5. Themethod as claimed in claim 4, wherein the act of placing the forked coilholder comprises mounting said forked coil holder to a shroudmanipulator.
 6. The method as claimed in claim 4, wherein the actplacing the forked coil holder comprises mounting said forked coilholder to a separate mounting device that is arranged to follow theposition of the shroud.
 7. The method as claimed in claim 4, wherein theact of placing the forked coil holder comprises mounting said forkedcoil holder to a sliding gate at the ladle.
 8. The method as claimed inclaim 1, further comprising: detecting turbulent flow, if any, insidethe shroud; and changing the frequency of the electromagnetic fieldgenerated by the transmitting coil in case of turbulent flow having beendetected.
 9. The method as claimed in claim 1, further comprisinggenerating, by means of said at least transmitting coil: anelectromagnetic field of alternating frequencies, or severalelectromagnetic fields with different frequencies.
 10. The method asclaimed in claim 1, wherein any induced voltage having a value outside adefined voltage range is indicative of the presence of slag, the methodfurther comprising: determining the flow of the molten metal passingthrough the shroud; and defining said voltage range depending on themagnitude of the determined flow of molten metal.
 11. The method asclaimed in claim 10, further comprising: defining a larger voltage rangeif it is determined that the magnitude of the flow of molten metal hasdecreased.
 12. The method as claimed in claim 10, wherein the act ofdetermining the flow of molten metal passing through the shroudcomprises: providing feedback from an opening position signal a slidinggate at the ladle and calculating the flow of molten metal from thesliding gate opening information.
 13. The method as claimed in claim 10,wherein the act of determining the flow of molten metal passing throughthe shroud comprises: measuring the rate of decrease in weight of theladle content and calculating the flow of molten metal from saidmeasured rate of decrease in weight.
 14. The method as claimed in claim10, wherein the act of determining the flow of molten metal passingthrough the shroud comprises: measuring the teeming rate in the tundishand calculating the flow of molten metal from said measured teemingrate.
 15. The method as claimed in claim 1, further comprising: coolingsaid transmitting and receiving coils.
 16. A device for detecting thepresence of slag in a shroud for guiding molten metal from a ladle to atundish, comprising: at least one transmitting coil for generating anelectromagnetic field to be entered into the shroud its contents; atleast one receiving coil for receiving the electromagnetic field thathas entered the shroud and its contents, and for generating an inducedvoltage, wherein said induced voltage is indicative of the presence orabsence of slag in said contents; a shroud manipulator adapted tomanipulate the shroud; a forked coil holder mounted to the shroudmanipulator and having at least two branches, a first branch carryingsaid at least one transmitting coil and a second branch carrying said atleast one receiving coil, the two branches being placeable in suchmanner that the shroud is located between them and that said at leastone transmitting coil and said at least one receiving coil are in astationary position in relation to the shroud.
 17. A device fordetecting the presence of slag in a channel for guiding molten metal,comprising: at least one transmitting coil for generating anelectromagnetic field to be entered into the channel and its contents;at least one receiving coil for receiving the electromagnetic field thathas entered the channel and contents, and for generating an inducedvoltage, wherein said induced voltage is indicative of the presence orabsence of slag in said contents; characterized by a coil holderarrangement that is adapted to hold said at least one transmitting coilin such manner that the generated electromagnetic field enters a shroudand its contents, the shroud being said channel for guiding molten metalfrom a a tundish, and to hold said at least one coil and said at leastone receiving coil in position in relation to the shroud.
 18. The deviceas claimed in claim 17, wherein said coils are in the form of toroids,wherein said coil holder arrangement is adapted to hold each toroid insuch manner that it surrounds the shroud.
 19. A device as claimed inclaim 17, wherein said coil holder arrangement comprises a forked coilholder having at least two branches, a first branch carrying thetransmitting coil(s) and a second branch carrying the receiving coil(s),the two branches being placeable in such manner that the shroud islocated between them.
 20. The device as claimed in claim 19, wherein theforked coil holder is adapted to be mounted to a shroud manipulator. 21.The device as claimed in claim 19, wherein the forked coil holder isadapted to be mounted to a separate mounting device which is arranged tofollow the position of the shroud.
 22. The device as claimed in claim19, wherein the forked coil holder is adapted to be mounted to a slidinggate at the ladle.
 23. The device as claimed in claim 19, wherein saidtwo branches are electrically isolated from each other.
 24. The deviceas claimed in claim 16, wherein any induced voltage having a valueoutside a defined voltage range is indicative of the presence of slag,the device further comprising: means for determining the flow of themolten metal passing through the shroud; and means for defining saidvoltage range depending on the magnitude of the measured flow.
 25. Thedevice as claimed in claim 24, wherein said means for determining theflow of the molten metal passing through the shroud comprises: a sensorfor sensing an opening position signal of a sliding gate at the ladle,and a processor for calculating the flow of molten metal from thesliding gate opening information.
 26. The device as claimed in claim 24,wherein said means for determining the flow of the molten metal passingthrough the shroud comprises: a measuring device for measuring the rateof decrease in weight of the ladle content, and a processor forcalculating the flow of molten metal from said measured rate of decreasein weight.
 27. The device as claimed in claim 24, wherein said means fordetermining the flow of the molten metal passing through the shroudcomprises: a measuring device for measuring the teeming rate in thetundish, and a processor for calculating the flow of molten metal fromsaid measured teeming rate.
 28. The device as claimed in claim 16,wherein the transmitting and receiving coils are provided withdirectional elements, such as a core, for directing the electromagneticfield towards and from the shroud.
 29. A casting plant, comprising aladle adapted to contain molten metal; a tundish adapted to receivemolten metal from the ladel; a shroud arranged between the ladle and thetundish, wherein molten metal is enabled to pass from the ladle, throughthe shroud, and to the tundish; and a device as claimed in claim 16.